Transmission apparatus



May 28, 1940. L. M. LEEDS 2,202,699

TRANSMISSION APPARATUS Filed Deo. 2l, 1935 2 Sheets-Sheet 1 Inventor: L.auan ce M. Leeds,

y Hwttorney.

May 28, 1940. M. LEEDS 2,202,699

TRANSMISSION APPARATUS Filed Dec. 2l, 1955 2 Sheets-Sheet 2 lll TInventor Laurence M. Leeds,

U H is Attorney.

Patented May 28, 1940 UNITED STATES PATENT OFFICE TRANSMISSION APPARATUSNew York Application December 21, 1935, Serial No. 55,584

Claims.

My invention relates to short wave apparatus and more particularly tofrequency selective transmission apparatus for operation at shortwavelengths.

One of the objects of my invention is to provide a short wavetransmission system having an improved band elimination characteristic.

A further object of my invention is to provide such a system capable ofefficiently transmitting currents of a desired frequency and of highlyattenuating currents of undesired frequency even though the frequenciesbe spaced relatively closely in the short wave range of the frequencyspectrum.

My invention is particularly adapted for use, for example, in policecommunication systems for efficiently separating oscillations to bereceived, for example, in an automobile receiver, from oscillationsradiated from a transmitter at the same time and from the sameautomobile and which may be connected, for example, to the same antenna.One range of frequencies commonly employed for such communicationextends approximately from thirty to forty-two megacycles. Since thereceiver on a police automobile, or at a police station, forsimultaneous transmission and reception must operate at a frequencydifferent from that at which the associated transmitter operates, andsince the frequency band is comparatively narrow, it is important thatmeans be associated with the receiver which will greatly attenuate thetransmitter frequency. In this way the spacing between transmitted andreceived frequencies may be reduced and the band may be more eicientlyutilized. An object of my invention is to provide improved means forthis purpose.

The use of concentrated inductances and capacitors connected inconventional frequency selective circuits are quite unsatisfactory foroperation at short wavelengths because of excessive attenuation at thedesired frequencies produced by reason of the high resistance of suchcircuit elements at the frequencies involved. In addition strayinductance and capacity undesirably affect the impedance valuesobtainable and thus reduce the selectivity of the apparatus and ingeneral detract from the quality of the filter characteristicobtainable.

It has been found in accordance with my invention that thesedifficulties are greatly reduced by the use of the standing wavetransmission line in the manner which I shall presently describe.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, both as to its organization and method of operationtogether with further objects and advantages thereof may best be (Cl.Z50-9) understood by reference to the following description taken inconnection with the accompanying drawings. Figs. 1 to 4 respectivelyillustrate the use of standing wave transmission lines of differentlength in accordance with my invention, and Figs. 5, 6, 7, and 8illustrate certain structural details of such a line when utilized inaccordance with my invention.

Referring to Fig. 1 I have indicated therein, a section of a concentrictransmission line comprising an outer conducting tube i and an innercentrally disposed conductor 2. If oscillations be impressed upon theconductor 2 of this concentric line, as indicated at 3, it will be seenthat a standing wave will be produced upon the line. This standingvoltage wave has a maximum at the right end of the line, since thetransmission line is there open circuited, a minimum value at a pointone-quarter of a wavelength of the impressed oscillations from the rightend, and a maximum at a half Wavelength from the right end, all asindicated by the curve 5. Such a standing wave is produced upon the lineirrespective of the frequency impressed, but the point upon thetransmission line at which the nodal point of the standing Wave existsis, of course, dependent upon the frequency of the standing wave.Accordingly, if the source 3 be one producing an undesired frequency,the receiving equipment may be connected to the transmission line at thenodal point of the undesired wave, as for example by conductors 4, andsubstantially no voltage of the undesired frequency will be impressedupon the receiving equipment. Voltage of any other frequency present maybe received.

Fig. 2 is similar to Fig. 1 with the exception that the transmissionline is represented as having a length equal to a full wavelength of thefrequency which it is desired to exclude from the receiving equipment.The standing wave of this undesired frequency is represented by thecurve 6 thereon. It will be observed that this Wave has two nodalpoints, one occurring at the point l, which is three-quarters of awavelength from the right end thereof and the other at the point 8,which is one-quarter of a wavelength from the right end thereof. Ofcourse, if the transmission line be longer these points occur at eachodd multiple of a quarter wavelength of the undesired oscillation fromthe open end of the transmission line. Curves 9 and lil represent thestanding waves of two frequencies respectively lowerV and higher thanthe undesired frequency and which may also be impressed on the line fromsource 3. It will be observed that the nodal points of these frequenciesoccur at points considerably displaced along the transmission line fromthe nodal points of the curve and further, that the amplitude oftheselatter oscillations at the points l' and ii corresponding to thenodal points of the undesired Wave are of considerable magnitude as isindicated at il and l2. Accordingly, if the receiving equipment beconnected between the central conductor 2 and the shield i at thesenodal points the undesired Wave 6 will be completely excluded therefromWhereas oscillations of the frequency corresponding to waves 9 and iiiwill be received.

It will further be noticed that the amplitude of oscillations cf thefrequency represented by the curves Q and i@ is greater at the point 'ithan at the point d. Accordingly, while the receiving equipment may beconnected at either of these points preferably it is connected at thepoint "i, l'n addition the amplitudes of oscillations represented bycurves Si and i il at the point 'i of the transmission line are fargreater than the amplitude of oscillations supplied to the line by thesource 3. Thus by suitable selection of the point of connection of thereceiving equipment the undesired wave may not only be excluded but asubstantial step up in voltage of the desired wave may be obtained.

Figs. 3 and 4 differ from Fig. i in that they represent transmissionlines in which the inner conductor is connected to the outer conductorat the right end thereof as indicated at i3. It will be seen on theselines that nodal points of the unn desired Wave represented by thecurves ifi occur at points spaced from the closed end iii by a halfWavelength of the undesired wave or any multiple thereof. Accordingly,these points are points at which a receiving apparatus may be connectedto receive oscillations of frequencies other than that which it isdesired to exclude.

Fig. 5 represents an application of my invention in which thetransmission line comprises a number of sections i5, itl, il", i8 oftransmission line of any desired length connected in series and foldedback upon each other` into a layer. Of course, if desired the shield maybe extended over the inner conductor at the ends of the sections in anysuitable manner. Oscillations of a frequency to be received areimpressed upon this transmission line from an antenna is. This antenna,as in the police application above referred to, in addition to itsfunction as a receptor of radiant energy which it is desired to receivemay also act as the radiating member for transmitting equipment, aportion of which is indicated at 20. This transmitting equipment mayoperate at a frequency different from that 'which it is desired toreceive through the transmisison line, and accordingly the receivingequipment, an input element of which is represented on the drawing bythe coil 2i, is connected to the transmission line at a point 22corresponding to a nodal point of Voltage of the frequency at which thetransmitting equipment 2li operates. This connection of the receivingequipment 2l may be made through a shielded cable upon which eitherstanding or travelling Waves occur. It will be seen that the innerconductor of the transmission line i5, iii, il, i3 is disconnected fromthe shield at the end and accordingly the point 22 is spaced from theend 2d by a distance along this line equal to a quarter of a wavelengthof the frequency at which the transmitter operates, or some suitable oddmultiple thereof. The different sections of the line may be folded backupon each other as indicated and the shields thereof securelyconductively bonded together and grounded as indicated at 225. ln thisway the line is prevented from itself acting as a receptor of radiantenergy and all of the energy impressed upon the line is received eitherfrom the antenna it or transmitter 2li and appears on the line asoscillations between the inner and outer conductors.

it has been found in accordance with my invention that such an equipmentoperates very efficiently at short Waves to exclude undesiredfrequencies. That is, the equipment has a highly desirable band passfrequency characteristic. In fact, it has been found that when operatingin the 30 to l2 megacycle band with equipment in accordance with myinvention adapted for police operation that oscillations radiated by atransmitter mounted on the automobile carrying my equipment could besatisfactorily eliminated from the receiving equipment by use of myinvention even though the frequency which it was desired to receive Wasspaced only approximately i megacycle from the frequency at which thetransmitter operated. By use of my invention even closer spacing isentirely practical.

Such a filter, it will be seen, possesses further advantages in themanner in which it may be mounted in an automobile. Since the outerconductor of the different sections is at ground potential andcompletely shields the inner conductor upon which the standing waveoccurs, it may be conveniently mounted permanently into the frame orbody of the automobile, or if desired it may be rolled up as indicatedin Fig. 7 and stored in the rear end trunk now commonly provided uponautomobiles.

In the case of a high pov/er transmitter, for example, Where one sectionof filter is insufficient to exclude the undesired frequency to theextent desired, a number of sections may be provided as shown in Fig. 6where two sections 2t and 21 are indicated as connected in cascade, theinput to section 2l being connected to a point on section 2&3 Where anodal point of the voltage which it is desired to exclude occurs, andsimilarly the receiving equipment being connected to a point on section2'? where a nodal point of the undesired wave occurs.

Fig. 8 represents in greater structural detail a two-section lter, suchas that shown in Fig. 6. It will be seen that the center conductor 2 ismounted within the shield i of the concentric transmisison line by meansof suitable insulating beads 2d of a type now commonly employed inconcentric transmission line practice. The length of the transmissionline of Fig. 8 comprises a filter section corresponding to the section26 of Fig. 6. The second section of the illter corresponding to section2l of Fig. 6 is then connected to the length by means of suitable "i"joints El, this connection, of course, being made at a nodal point ofthe undesired wave. Since in this figure the inner conductor 2 isindicated as connected to the shield at the point the T connection is,of course, made at a distance from the end 32 equal to a halfwavelength, or a suitable multiple thereof, of the oscillations which itis desired to exclude. It will be noted that the length iii. indicatedas broken at the point 33 to indicate additional length. Similar breaksfor the same purpose are indicated at 35i in the second section of thelter. The receiver is connected to the second section of the filter bymeans of the l joint connection The transmission line constructed asillustrated in the drawings7 of course, has an input impedance at theantenna end, as in Figs. 5 and 6 for example, which is high as comparedwith the impedance of the antenna which might be utilized at I9 in thewave band referred to. This input impedance, may for example, be in theneighborhood of several thousand ohms whereas the impedance of theantenna utilized upon an automobile and connected as shown in Fig. maybe about thirty-six ohms. In a particular transmission line of threequarters of an inch in diameter and a multiple of a quarter wavelengthin length the input impedance was of the order of 10,000 ohms. This, ofcourse, aids materially in eliminating from the receiver the transmittedwave. The transmitted power of course divides between the antenna andtransmission line in inverse proportion to these impedances and hence itis but a small portion of it which penetrates the transmission line. Theline, of course, may be constructed to have as large input impedance aspracticable to aid in eliminating the transmitted wave from thereceiver.

While in the different iigures of the drawings I have shown lterscomprising transmission lines of the concentric type, it will beunderstood that I do not wish to be limited thereto since open widetransmission lines comprising merely a pair of parallel conductors maylikewise be employed. The concentric line, however, has decidedadvantages over an open, or unshielded, wire line. An unshielded line,for example, produces a certain amount of radiation with the result thatit possesses a certain amount of radiation resistance. This in turnresults in greater voltage of the undesired frequency at the points ofminimum voltage of the undesired wave. The concentric line, on the otherhand, produces substantially no radiation and its radiation resistanceis substantially zero with the result that substantially no voltage ofthe undesired frequency exists at the nodal points of the undesiredwave.

While I have shown particular embodiments of my invention it will beunderstood that I do not wish to be limited thereto since differentmodilications may be made without departing from the spirit and scope ofmy invention. I contemplate by the appended claims to cover any suchmodifications as fall within the true spirit and scope of my invention.

Wha I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a short wave radio signal system comprising a short wave antenna,and a transmitter and receiver connected thereto and adapted foroperation at different wavelengths, a standing wave transmission lineconnected to said antenna to have oscillations of each of saidwavelengths at which said transmitter and receiver operate impressedbetween the two conductors thereof at the end of said line adjacent saidantenna, means to excite on said line standing waves of each of saiddifferent wavelengths and to produce on said line a node of voltage ofthe wavelength at which the transmitter operates between said conductorsat a certain point where exist substantial voltages of the wavelength atwhich the receiver operates, and means to connect said receiver betweensaid conductors at said point.

2. In a short wave radio receiving system comprising an antenna forintercepting waves of a desired short wavelength, said antenna havingwaves impressed thereon of different undesired short wavelength, and areceiving device, a high frequency standing wave transmission lineconnected to said antenna to have said waves of each of said wavelengthsimpressed on the opposite conductors thereof in opposite phase, meansincluding said antenna to produce in said transmission line a standingwave of said undesired wavelength having a plurality of nodal voltagepoints thereon and to cause the voltage wave of the desired wavelengthto have an amplitude at one of said nodal points larger than the voltageof the desired wavelength at said antenna, and means to connect saidreceiving device between said conductors at said point.

3. In a short wave radio signal system comprising a short wavetransmitter, a short wave receiver and a short wave antenna connected toboth said transmitter and receiver, a standing wave transmission lineconnected to said antenna and having a length equal to a multiple of aquarter of a wavelength of the wave radiated by said antenna, meansincluding said antenna to produce a standing wave of said wavelength onsaid transmission line, means to connect said receiver across said lineat a point where said standing wave has a nodal point of voltage, saidtransmission line having an input impedance substantially higher thanthe impedance of said antenna whereby but a small portion of the energyproduced by said transmitter penetrates said transmission line.

4. In a short wave radio receiving system comprising an antenna forintercepting waves of a desired short wavelength, an associatedtransmitter, said antenna having Waves impressed thereon of differentundesired short wavelength from said associated transmitter, and a loaddevice for utilizing oscillations of said desired short wavelength, aband elimination filter comprising a pair of concentric conductorshaving a length equal to a multiple of a quarter of .a wavelength ofsaid undesired wavelength, means to connect said conductors at adjacentends thereof to said antenna, means including said antenna to produce astanding wave in said pair of conductors having a nodal point of voltageof said undesired wavelength, and means to connect said load devicebetween said conductors at said nodal point of voltage of said undesiredwavelength, the impedance of said antenna being low as compared with theinput impedance of the line comprising said pair of concentricconductors whereby but a small portion of the energy of said undesiredwavelength penetrates said line.

5. In a short wave radio apparatus for mounting and operation on a motorvehicle comprising a short wave transmitter and a short wave receiveradapted to operate at different wavelengths, and a short wave antennamounted on said motor vehicle and connected to both said transmitter andreceiver, a standing wave transmission line connected to said antennaand having a length equal to a multiple of a quarter of a wavelength ofthe wave radiated by said antenna, means including said antenna toproduce a standing wave of said wavelength on said transmission line,and means to connect said receiver across said line at a point wheresaid standing wave has a nodal point of voltage.

LAURANCE M. LEEDS.

